1. Field of the Invention
The present invention relates to a transmitter, more particularly to phase adjustment of a transmitter chain comprising a quadrature modulator, a variable gain amplifier, an up-converter, and a variable gain power amplifier.
The present invention further relates to a phase adjuster, to a method of adjusting an overall phase of a transmitter chain, and to a communication device with a phase adjuster in a transmitter chain.
Such a transmitter can be a transmitter in a full-duplex direct sequence spread-spectrum CDMA system, or any other suitable system with a high dynamic range transmitter output signal.
2. Description of the Related Art
Full-duplex CDMA systems are known in which transmitters have a high dynamic range output signal, typically a dynamic range of more than 70 dB. Newer CDMA systems, but also other systems, are referred to as linear modulation systems in which information is carried on amplitude as well as on phase. In such systems non-linear amplifiers cannot be adopted because the amplitude of a modulated signal varies within a wide range, i.e., the modulated peak signal envelope exhibits large fluctuations. Therefore, amplifiers in a transmitter chain need to be very linear. Particularly for a variable gain power amplifier that amplifies an up-converted signal of high frequency, e.g. in a GHZ band, meeting linearity requirements over a wide range is not easy. Typically, such variable gain power amplifiers are optimized in efficiency at maximum signal output but have poor efficiency at low signal output.